Thermal battery cells containing cathode materials in low-melting nitrate electrolytes

ABSTRACT

The addition of cathode materials comprising Cu ++ , Fe +++ , Cr +++   or Au +++ , in the form of salts such as the nitrate or halide, e.g. Fe(NO 3 ) 3  or CuCl 2 , to low melting nitrate electrolyte cells increases cell potential. Other ions such as Co ++ , Eu +++ , La +++ , Ni ++ , Mn ++ , Ce +++ , Pr +++ , Nd +++ , Gd +++ , Sm +++   and Tb +++ , in the form of salts thereof, can also be used, but yield smaller cell potentials. Such cathodic materials in the form of a suitable salt, such as a nitrate or halide, e.g. Fe(NO 3 ) 3  or CuCl 2 , are added to low melting fused nitrate electrolytes, e.g. a LiNO 3 , KNO 3  mixture, in a concentration sufficient to increase cell potential, using Li or Ca anodes. A suitable metal current collector such as a Ni screen can be used as a cathode. The above cathodic materials can be used in conjunction with other cathodic materials such as AgNO 3 , which undergoes reduction to the free metal.

The invention described herein may be manufactured and used by or forthe government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of electrochemistry, and moreparticularly relates to thermally activated electrochemical cells havinga low melting nitrate electrolyte containing novel cathode materials,resulting in an increase in cell potential.

2. Description of the Prior Art

Thermally activated electrochemical cells or batteries have been usedquite extensively in military applications, such as a power source forarming devices, because of their long shelf life and compactness, andcapability of withstanding shock and vibration. Batteries of this typetypically include an electrolyte which, under normal storage conditions,is solid and does not conduct electricity. When the battery and/or theelectrolyte is heated to a predetermined temperature, as by igniting abuilt-in pyrotechnic heat source such as an electric match, squib orpercussion primer, the electrolyte, upon changing to a molten state,becomes conductive and ionically connects the electrodes to provide thedesired electromotive force.

Nitrate salts have been proposed for use in thermal batteries because oftheir low melting points. See U.S. Pat. No. 4,260,667 to Miles andFletcher. For example, potassium nitrate-lithium nitrate (KNO₃ -LiNO₃)mixtures melt at temperatures as low as 124° C. The use of a lowermelting electrolyte can shorten a battery's activation time and reducethe weight of heat sources and insulation.

A particular problem area of thermal battery cells is the lack of highperformance cathode materials. Adding silver salts to electrolytes ascathode materials to improve cell potentials has previously beenunsuccessful because the cathodic reactions involve the reduction ofsilver ions to the free metal in reversible electrode reactions, such as

    AgNO.sub.3 +e-⃡Ag+NO.sub.3

Consequently, no net cathodic current can flow in such cells at cathodepotentials more positive than the silver ion/silver reversiblepotential. Additionally, the added silver salts migrate and diffuse tothe anode and form silver metal films on the anode surface thatinterfere with cell operation. Many divalent and trivalent metal ionscannot be used at high temperatures in molten nitrate electrolyte cellssince they react rapidly with the nitrate melt to form the metal oxide.

U.S. Pat. No. 4,416,958 to Miles and Fletcher discloses a thermallyactivated electrochemical cell having a low melting point electrolyte.The electrolyte is composed of a layer of a mixture of lithiumperchlorate and lithium nitrate adjacent the anode and of a layer of amixture of lithium perchlorate, lithium nitrate, and silver nitrateadjacent to the cathode of the cell.

The article "Cyclic Voltammetric Studies of Nitrato Complexes of VariousMetal Ions in Molten LiNO₃ +KNO₃ at 180° C." by M. H. Miles et al, J.Electoanal Chem., 221 (1987) 115-128, discloses addition of variousmetal ions such as Co⁺⁺, Cu⁺⁺, Au⁺⁺⁺, Mn⁺⁺, La⁺⁺⁺, and Ce⁺⁺⁺ ions tomolten nitrates, and the effect of such additions.

However, the above article does not disclose or teach the application ofthe principles or concepts that are disclosed therein to thermalbatteries, particularly employing lithium or calcium anodes.

One object of the invention accordingly is the provision of an improvedthermal electrochemical cell.

Another object is to provide a novel thermal electrochemical cellutilizing low melting nitrate electrolytes.

A still further object is the provision of improved thermalelectrochemical cells incorporating certain cathode materials in theelectrolyte.

Yet another object is to provide thermal electrochemical cells havingnitrate electrolyte and containing certain metal salts as cathodicmaterial, to increase the potential of the cell.

SUMMARY OF THE INVENTION

According to the present invention, it has now been found that additionof certain metal ions in the form of salts to low melting nitrateelectrolytes in an electrochemical cell containing a lithium or calciumanode, when activated by heating, produces cathodic currents at cathodepotentials significantly more positive than the reversible potential forthe metal ion/metal reaction, as in the case of silver salts, as notedabove, thereby producing greater cell potentials than previouslyachieved.

The greatest increase in cell potential is accomplished employing acathode material comprising Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺ or Au⁺⁺⁺ ions and a lowmelting fused salt nitrate electrolyte such as the eutectic mixture ofLiNO₃ and KNO₃. Other ions such as Co⁺⁺, Eu⁺⁺⁺, La⁺⁺⁺, Ni⁺⁺, Mn⁺⁺,Ce⁺⁺⁺, Pr⁺⁺⁺, Nd⁺⁺⁺, Gd⁺⁺⁺, Sm⁺⁺⁺ and Tb⁺⁺⁺ can also be used, but yieldsmaller cell potentials. Such ions are added in the form of a suitablesalt of such metal ions to the low melting fused nitrate saltelectrolyte. The cathode materials can be added directly to the fusedsalt electrolyte at various concentrations, as noted hereinafter.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The cathode material comprising one or more of the above ions, ormixtures thereof, is added to the low melting fused nitrate electrolytein the form of a salt which is soluble in the electrolyte, such as anitrate or a halide, e.g. chloride, salt, and diffuses in theelectrolyte.

The following metal salts have been found useful as a source of thedesired metal ions in the nitrate melts: CuCl₂, CuBr₂, FeCl₃, Fe(NO₃)₃,CrCl₃, AuCl₃, Co(NO₃)₂, EuCl₃, LaCl₃, Ni(NO₃)₂, MnCl₂, CeCl₃, PrCl₃,NdCl₃, GdCl₃, Sm(NO₃)₂, and TbCl₃. Any of these cathode materials, suchas CuCl₂ or Co(NO₃)₂, can be added directly to the fused nitrateelectrolyte in concentrations ranging from about 2×10⁻⁴ to about 2×10⁻¹m(molal). The cathodic current increases directly with increase inconcentration.

The electrochemical cells of the present invention employ low meltingnitrate salts. Generally, mixtures of nitrate salts are employed whichmelt at temperatures not substantially greater than 200° C. Specificexamples of such nitrate electrolyte salts are mixtures such as LiNO₃/KNO₃ (42-58 mole %), LiNO₃ /NaNO₃ (56-44 mole %), LiNO₃ /NaNO₃ /KNO₃(37.5-18-44.5 mole %) and LiNO₃ /RbNO₃ (30-70 mole %). Such nitrateelectrolytes can also include a nitrite component such as NANO₂, e.g.the mixture LiNO₃ /KNO₂ (40-60 mole %). All of such mixtures havemelting points below 200° C.

The cells of the present invention preferably are operated attemperatures not greater than about 200° C. Higher temperatures causedecomposition of the nitrate melts in the presence of the added metalions. Thus, cells of this invention are operated at temperatures between124° C. and 200° C. The performance of the cathode materials varies indifferent nitrate electrolyte melts and at different temperatures.

Although applicant is not certain as to the particular theory ofoperation of the above cathodic materials in the low melting nitrateelectrolytes, the increase in the cell potential obtained employing thecathode materials of the invention is achievable because there is noreversible reduction of metal ions to the free metal as in the case ofthe silver salts. Instead, the metal salt cathode materials of thepresent invention combine with the low melting nitrates to form nitratocomplexes of metal ions which are reduced in irreversible cathodicreactions.

The metal salts comprising the cathode material according to theinvention, can be employed in electrochemical cells having Li or Caanodes because the metal ion of the cathode material is not reduced tothe free metal. This prevents any metal film from forming on the anodesurface. Instead, the nitrato complexes of the metal ions are reduced tothe metal oxide or oxide species such as FeO⁺. Many of the metal oxideproducts thus produced in the present invention are sufficiently solublein the electrolyte melt, and do not passivate the cell electrodes, andthe cathode materials hereof can accordingly be allowed to mix with theelectrolyte throughout the entire cell.

The metal salts of which cathode materials are comprised can be used inconjunction with other cathodic materials such as silver salts, e.g.AgNO₃, that undergo reduction to silver metal. Under thesecircumstances, a small concentration of silver salt not to exceed theconcentration of the metal salt cathode material hereof should beemployed, e.g. a proportion of about 0.01 to about 1 mole of silver saltper mole of metal salt cathode material, and it is essential that thesilver ion be kept near the cathode or cathode collector, so it does notplate out on the anode. Many metal ion additives of the presentinvention, Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺, and Au⁺⁺⁺, give cathodic reactionspositive to the Ag⁺ /Ag cathode reaction.

In the electrochemical cell of the present invention the low meltingnitrate electrolyte containing the metal salt cathode materials hereofis disposed between a lithium or calcium anode, and a cathode, which canbe in the nature of a metal current collector such as a nickel screen,with electrical connections to the anode and the cathode. However, ifdesired, the metal salt cathode material can be used as a solid cathodelaver in the electrolyte in solid form, spaced from the anode, andadjacent to the cathode current collector, instead of being addeddirectly to the electrolyte melt.

According to a preferred embodiment of the invention, providing athermal battery cell with a lithium anode, a nickel screen cathodecurrent collector spaced from the anode, and a LiNO₃ /KNO₃ electrolytecontaining CuCl₂ cathode material in a concentration of 0.02 m withrespect to the electrolyte, the electrolyte can be provided as a discsuch as of fiberglass filter paper with the electrolyte containing thecathodic materials absorbed thereon. Such electrolyte can be prepared bydipping the disc into the molten electrolyte having the CuCl₂ cathodematerial diffused therein, removing and allowing the electrolyte tosolidify. The treated fiberglass discs are then placed with their flatsurfaces adjacent to each other and sandwiched between the anode andcathode to form the cell as described above.

The following are examples of practice of the invention:

EXAMPLE 1 CuCl₂ as Cathode Material

Cyclic voltammetric studies at 50 mV/s (millivolts per second) of 0.020m CuCl₂ in LiNO₃ /KNO₃ (42-58 mole %), electrolyte m.p.=124° C.) at 180°C. show dramatic increases in the electrochemical reactivity of theLiNO₃ /KNO₃ melt. Reduction waves for the melt begin near +0.3 V (vs.Ag⁺ /Ag). From the results of the cyclic voltammetric studies, athermally activated electrochemical cell constructed having a Li anode,LiNO₃ /KNO₃ electrolyte (mole fraction KNO₃ =0.58, m.p.=124° C.) and aCuCl₂ cathode will have an open circuit potential (Eoc)=3.7 V at 180° C.Current densities of 4 mA/cm² can be achieved for 0.02 m CuCl₂ anddensities of 20 mA/cm² can be achieved for 0.1 m CuCl₂. A cell potentialof 3.4 V at 20 mA/cm² can be achieved for 0.1 m CuCl₂.

EXAMPLE 2 FeCl₃ as Cathode Material

Cyclic voltammetric studies at 50 mV/s of 0,020 m FeCl₃ in LiNO₃ /KNO₃(42-58 mole %) electrolyte, m.p.=124° C.) at 180° C. show dramaticincreases in the electrochemical reactivity of the LiNO₃ /KNO₃ melt.Reduction waves for the melt begin near +0.6 V (vs. Ag⁺ /Ag). From theresults of the cyclic voltammetric studies, a thermally activatedelectrochemical. cell constructed having a Li anode, LiNo₃ /KNO₃electrolyte (mole fraction KNO₃ =0.58, m.p=124° C. and a FeCl₃ cathodematerial would have an Eoc=4.0 V at 180° C. Current densities of 2mA/cm² can be achieved for 0.02 m FeCl₃. A cell potential of 3.8 V at 10mA/cm can be achieved for 0.1 m FeC13.

EXAMPLE 3

AuCl₃ as Cathode Material

From cyclic voltammetric studies similar to Examples 1 and 2, employingthe same nitrate electrolyte and the same Li anode as seen in Examples 1and 2, but using AuCl₃ as cathode material, the cathodic wave begins at+0.8 V (vs. Ag⁺ /Ag). As set forth in the previous examples, a cellconstructed having 0.02 m AuCl₃ will have an Eoc=4.2 V at 180° C. A cellpotential of 3.8 V can be achieved at a current density of 10 mA/cm² for0.1 m AuCl₃.

EXAMPLE 4 Ni(NO₃)₂ as Cathode Material

From studies similar to Examples 1 and 2, a new cathodic wave begins at-0.3 V (vs. Ag⁺ /Ag) for 0.02 m Ni(NO₃)₂ added to the LiNO₃ /KNO₃ meltat 180° C. Therefore, a thermally activated cell constructed with alithium anode and 0.02 m Ni(NO₃)₂ in LiNO₃ /KNO₃ will have an Eoc=3.1 Vat 180° C. A cell potential of 2.4 V can be achieved at 25 mA/cm² for0.1 m Ni(NO₃)₂.

EXAMPLE 5 Co(NO₃)₂ as Cathode Material

From cyclic voltammetric studies similar to Examples 1 and 2, thecathodic wave begins at -0.4 V (vs. Ag⁺ /Ag) with 0.02 m Co(NO₃)₂ addedto the LiNO₃ /KNO₃ melt at 180° C. Therefore, a cell constructed with alithium anode and 0.02 m Co(NO₃)₂ will yield an Eoc=3.0 V at 180° C. Acell potential of 2.5 V can be achieved at a current density of 20mA/cm² for 0.1 m Co(NO3)₂.

EXAMPLE 6 EuCl₃ as Cathode Material

From cyclic voltammetric studies similar to Examples 1 and 2, a newcathodic wave begins at -0.5 V (vs. Ag⁺ /Ag) with 0.02 m EuCl₃ added tothe LiNO₃ /KNO₃ melt at 180° C. Therefore, a cell constructed with alithium anode and 0 02 m EuCl₃ will have an Eoc=2.9 V at 180° C. A cellpotential of 2.4 V can be achieved at a current density of 10 mA/cm² for0.1 m EuCl₃.

EXAMPLE 7 MnCl₂ as Cathode Material

From cyclic voltammetric studies similar to Examples 1 and 2, the newcathodic wave begins at -0.6 V (vs. Ag⁺ /Ag) when 0.02 m MnCl₂ is addedto the LiNO₃ /KNO₃ melt at 180° C. Therefore, a cell constructed with alithium anode and 0.02 m MnCl₂ in LiNO₃ /KNO₃ will have an Eoc=2.8 V. Acell potential of 2.5 V can be achieved at a current density of 25mA/cm² for 0.1 m MnCl₂ as the active cathode material.

The present invention has the novel feature of producing cathodiccurrents at potentials significantly more positive than the potential ofcertain electrode reactions, thereby producing greater cell potentialsthan could be previously achieved.

This is believed due to the formation of nitrato-complexes with themetal ions that allows the nitrate ions to be reduced more readily.Negative electrons can be transferred more readily to nitrate ions thatare associated with positive metal ions. The complexed nitrate ions canthen be reduced at potentials more positive than potentials wherereduction of metal ions occur.

The effects of increased cell potential have been observed using thecathode materials of the invention at various concentrations in lowmelting nitrate electrolytes by adding the cathode materials directly tothe electrolyte melt. The use of the cathode materials of the inventiondoes not interfere with the lithium or calcium anodes employed. Aspreviously noted, the use of the cathodic materials of the invention canbe practiced in conjunction with other cathodic materials such as silversalts, which undergo reduction to the free metal.

Since various changes and modifications can be made in the inventionwithout departing from the spirit of the invention, the invention is notto be taken as limited except by the scope of the appended claims.

What is claimed is:
 1. A thermal electrochemical cell comprisinga low melting nitrate electrolyte which is a non-conductive solid at ambient temperature and is capable of becoming an ionically conductive liquid upon being heated above its melting point, a cathode material in said electrolyte, said cathode material comprising a metal ion selected from the group consisting of Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺, Au⁺⁺⁺, Co ⁺⁺, Eu⁺⁺, La⁺⁺⁺, Ni⁺⁺, Mn⁺⁺, Ce⁺⁺⁺, Pr⁺⁺⁺, Nd⁺⁺⁺, Gd⁺⁺⁺, Sm⁺⁺⁺ and Tb⁺⁺⁺ ions and mixtures thereof, and an anode in contact with said electrolyte, and selected from the group consisting of Li and Ca anodes.
 2. The thermal cell of claim 1, said nitrate electrolyte having a melting point and being capable of activation at temperatures not greater than about 200° C.
 3. The thermal cell of claim 1, said nitrate electrolyte being selected from the group consisting of mixtures of (1) LiNO₃ and KNO₃, (2) LiNO₃ and NaNO₃, (3) LiNO₃, NaNO₃ and KNO₃ and (4) LiNO₃ and RbNO₃.
 4. The thermal cell of claim 3, said nitrate electrolyte being a LiNO₃, KNO₃ mixture.
 5. The thermal cell of claim 1, said nitrate electrolyte including a nitrite.
 6. The thermal cell of claim 1, wherein said cathode material is a salt containing said metal ion.
 7. The thermal cell of claim 2, wherein said cathode material comprises a metal ion selected from the group consisting of Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺ and Au⁺⁺⁺.
 8. The thermal cell of claim 6, wherein said cathode material is selected from the group consisting of nitrate and halide salts of said metal ion.
 9. The thermal cell of claim 6, wherein the concentration of said cathode material in said electrolyte ranges from about 2×10⁻ to about 2×10⁻ molal.
 10. The thermal cell of claim 7, wherein said cathode material is a soluble salt of said metal ion selected from the group consisting of nitrate and chloride salts.
 11. The thermal cell of claim 6, wherein said electrolyte is LiNO₃ /KNO₃, and wherein the concentration of said cathode material is said electrolyte ranges from about 2×10⁻⁴ to about 2×10⁻¹ molal.
 12. The thermal cell of claim 6, and including a cathode current collector in contact with said electrolyte and spaced from said anode.
 13. The thermal cell of claim 6, said metal salt cathode material comprising a solid layer in said electrolyte spaced from said anode.
 14. The thermal cell of claim 9, said cathode material also including a silver salt in a proportion of about 0.01 to about 1 mole per mole of said salt containing said metal ion.
 15. The thermal cell of claim 10, wherein said electrolyte is LiNO₃ /KNO₃, and wherein the concentration of said cathode material in said electrolyte ranges from about 2×10⁻⁴ to about 2×10⁻¹ molal.
 16. In a thermal electrochemical cell comprising an anode, a cathode and an electrolyte disposed between said anode and said cathode,a low melting nitrate electrolyte which is a non-conductive solid at ambient temperature and is capable of becoming an ionically conductive liquid upon being heated above its melting point, said nitrate electrolyte having a melting point and being capable of activation at temperatures not greater than about 200° C., a cathode material diffused in said electrolyte, said cathode material comprising a metal ion selected from the group consisting of Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺, Au⁺⁺⁺, Co⁺⁺, Eu⁺⁺⁺, La⁺⁺⁺, Ni⁺⁺, Mn⁺⁺, Ce⁺⁺⁺, Pr⁺⁺⁺, Nd⁺⁺⁺, Gd⁺⁺⁺, Sm⁺⁺⁺ and Tb⁺⁺⁺, and mixtures thereof, and an anode in contact with said electrolyte, and selected from the group consisting of Li and Ca anodes.
 17. The thermal cell of claim 16, and including a cathode current collector in contact with said electrolyte and spaced from said anode.
 18. The thermal cell of claim 17, said nitrate electrolyte being selected from the group consisting of mixtures of (1) LiNO₃ and KNO₃, (2) LiNO₃ and NANO₃, (3) LiNO₃, NaNO₃ and KNO₃ and (4) LiNO₃ and RbNO₃ ; and said cathode material is a salt containing said metal ion.
 19. The thermal cell of claim 17, said nitrate electrolyte being a LiNO₃, KNO₃ mixture, and wherein said cathode material is a soluble salt of said metal ion selected from the group consisting of nitrate and chloride salts.
 20. The thermal cell of claim 19, wherein the concentration of said cathode material in said electrolyte ranges from about 2×10⁻⁴ to about 2×10⁻¹ molal. 